In recent years, acrylic fibers have attracted commercial interest and attention as industrial materials, such as an asbestos-substitutive fiber, a heat insulating fiber, a heat resistant fiber, a cement reinforcing fiber, and a fiber for specialty paper, in addition to their usual use as clothing materials. The acrylic fibers for industrial use should be in the form of short fibers.
In the prior art techniques, polyacrylonitrile (PAN) is first dissolved in an appropriate solvent to form a spinning solution (dope) which is then subject to wet- or dry-spinning and subsequent drawing to produce a filament. In particular, PAN has similar properties to stiff chains, since the molecular chains of PAN are twisted to form an irregular helix due to the strong polarity of the nitrile groups in the side chains thereof. See, W. R. Krigbaum et al., Journal of Polymer Science, Vol. XLIII, pp. 467-488, 1960. When adding a strong polar solvent, such as dimethylformamide, dimethylacetamide, dimethylsulfoxide, an aqueous NaSCN solution, an aqueous ZnCl.sub.2 solution, and an aqueous HNO.sub.3 solution, the nitrile groups of PAN attract the molecular chains of the solution to couple therewith even at ambient temperature, and thereby the molecular chains are broken to form a fluidizable solution. Upon spinning the resulting solution through a small opening of a spinneret and subsequent removal of the solvent, PAN is solidified to form a filament. The filament thus obtained must be cut into a desired length to produce a staple fiber.
However, the solvents used are now recognized as a causative substance which may contribute to environmental pollution. Moreover, the complicated steps of extracting, recovering and purifying the solvents, as well as the maintenance of anti-pollution facilities, increase the production cost. Further, the filament thus formed appears to be a fiber, but it still remain substantially unoriented. Accordingly, the filaments thus obtained must be subject to drawing in a high stretch ratio of 5 to 30 in order to afford a complete fibrous structure in which the molecular chains are arranged in parallel with the axis of the fiber. This may also increase production costs.
In the case of an acrylic fiber having large surface areas, the process for manufacturing the same involves the more complicated steps of providing a spinning solution, spinning the solution, solidifying the spun filament, removing and recovering the solvent used, drawing and cutting the filament, fibrillizing the resulting fiber, and so forth.
In general, the acrylic fibers prepared by the prior art techniques are inadequate as spun yarns due to their poor elasticity and slippery surface. Further, they are not satisfactory in terms of their reinforcing, heat insulating, and binding properties which are required of an industrial material.
As an attempt to solve the problems encountered in the prior art techniques mentioned above, it has been suggested to use water in place of the hazardous solvents. Most such processes involve heating a hydrate of PAN to form a PAN/H.sub.2 O melt, and spinning the melt followed by drawing to give a PAN fiber. For example, U.S. Pat. No. 2,585,444 discloses that a PAN fiber can be produced by heating a PAN hydrate containing 30 to 85% water (by weight) above the melting point of the PAN hydrate to give a melted fluid and then melt-spinning the resulting fluid. U.S. Pat. Nos. 3,896,204 and 3,984,601 disclose a process in which a PAN hydrate containing about 20 to 30% water (by weight) is heated at a temperature ranging from 170.degree. to 205.degree. C. and the resulting amorphous melt is then subject to spinning and drawing in a stretch ratio of above 5 to give a fiber. U.S. Pat. Nos. 3,991,153 and 4,163,770 disclose a process in which a PAN hydrate containing 10 to 40% water (by weight) is heated and spun at a temperature above the melting point of the hydrate, that is, a temperature above which the melt forms an amorphous single phase, and the spun filament is then subject to drawing in a stretch ratio of 25 to 150 within a pressure vessel.
As explained above, the prior art processes involve a step of spinning a PAN/H.sub.2 O melt. However, since the spinning is carried out within the temperature range at which the melt exists in a random amorphous state, fibers in which the molecular chains of PAN are highly oriented cannot be obtained without a subsequent step of drawing in a high stretch ratio.
U.S. Pat. Nos. 3,402,231, 3,774,387 and 3,873,508 disclose a process in which a PAN melt containing at least 50% water (by weight) is first prepared at about 200.degree. C., and the resulting melt is then spun to form fibers. However, such large amounts of water contained therein and such high temperatures provide a PAN/H.sub.2 O melt in a random, amorphous form. The filaments obtained from the melt have a profile of fibers, but are, in reality, no more than a continuous foam which does not possess any oriented molecular chains nor fibrous structures.
British Patent No. 1,327,140 discloses that fibrils can be prepared by premolding PAN at an elevated temperature under high pressure followed by solid extrusion. However, it is hard to obtain a fibril of greater than several tens of millimeters in length by this prior art process. Furthermore, the fibril obtained by the process is discolored dark brown, being valueless for use in clothing.
We, the present inventors, in the course of undergoing an extensive study of a two-component system comprising PAN and water, have unexpectedly found that a PAN/H.sub.2 O mixture forms an amorphous melt at a temperature range above the melting point of the mixture. The melt, even if cooled to temperatures below the above melting temperature, is not solidified and still maintains its supercooled, melted state until the cooling temperature arrives at a selected temperature range. When further cooled to the temperature below the solidifying temperature (T.sub.c), the melt is crystallized and is returned to its original state. However, when the PAN/H.sub.2 O melt is cooled to form the supercooled state at a temperature below the melting point, the melt forms a gel crystal having a molecular order unlike the amorphous melt formed above the melting temperature. The gel crystal allows PAN to easily obtain a molecular orientation upon extrusion. The phenomenon that PAN, together with water, forms a gel crystal has first been found by the present inventors. It appears that in the gel crystal, the PAN molecular chains, together with water molecules, form innumerable, fine units having a certain order, and the units are arranged in three dimension so as to form a regular phase of super lattice structures which allow the molecules to be easily arranged.
The molecular chains of PAN in the gel crystalline state have a self-orientating property. Thus, if some weak directional shear forces are applied to the PAN/H.sub.2 O melt, the PAN molecules easily form a highly-oriented fibrous structure. In other words, if the gel crystal is extruded, the PAN molecular chains are aligned, while water contained in the melt is spontaneously expelled out of the system. As water is expelled, the PAN molecules are extended and gathered in parallel with each other so that a fiber structure is formed, thereby producing highly-oriented fibers even without a separate drawing process.
U.S. patent application Ser. No. 07/709,872 which was filed oil Jun. 4, 1991 in the name of the present inventors and is still pending, discloses pulp-like acrylic short fibers, prepared by simple extrusion of a PAN/H.sub.2 O melt in gel crystalline state and mechanically beating the resulting extrudate. The fibers are featured by having a highly-oriented fibril structure, a thickness distribution of 0.1 to 10 .mu.m, and a length distribution of 0.1 to 100 mm.
U.S. patent application Ser. No. 08/064,345 filed on May 20, 1993, which is a file wrapper application of U.S. Ser. No. 07/804,457 filed on Dec. 10, 1991, by the present inventors, shows a heat- and chemical-resistant, pulp-like, acrylic short fiber featuring a thickness distribution of 0.1 to 50 .mu.m, a length distribution of 1 to 20 mm, a thermal transitional temperature of above 200.degree. C., and a solubility of less than 5% in dimethyl formamide at room temperature. According to the invention disclosed in the above pending application, the extrudate is formed from gel-crystalline PAN/water without spinning, and the extrudate is then subject to heat stabilization.